1. Technical Field
The present invention relates to photoperiodic circuit control apparatus for stabilizing the amplification in photoperiodic circuits.
2. Background Art
FIG. 7 shows a conventional photoperiodic circuit. In this diagram, the photoperiodic circuit 103 comprises a rare-earth-doped fiber 1, a wavelength multiplexing light mixer/divider 2, an optical switch 3, an light divider 4, a delay fiber 5, an optical isolator 6, a beam input end 7 and a beam output end 8. The excitation light source drive circuit 9 drives the excitation light source 10 so that the excitation light source 10 outputs a beam having a constant intensity.
Then, if an optical signal is inputted into the beam input end 7, then the optical signal becomes periodic, and circulates around the photoperiodic circuit in the following order: light divider 4.fwdarw.delay fiber 5.fwdarw.optical isolator 6.fwdarw.rare-earth-doped fiber 1.fwdarw.wavelength multiplexing light mixer/divider 2.fwdarw.optical switch 3.fwdarw.light divider 4. Additionally, while the light is circulating around the photoperiodic circuit 103, a portion of this circulating light is outputted to the beam output end 8 through the light divider 4. The pulse width of the above-mentioned optical signal is shorter than the time required for one circuit (the period).
In this case, the time from the point at which the inputted optical signal begins to circulate until the time at which the next inputted optical signal begins to circulate is called the frame period. The delay fiber 5 is provided in order to perform this type of time adjustment. When the photoperiodic circuit 103 is cut off by the optical switch 3, the light circulating in the photoperiodic circuit stops circulating.
If the amplification rate of the circulating light due to the rare-earth-doped fiber 1 is equal to the attenuation rate of the circulating light due to the circulation around the photoperiodic circuit, then the light circulating in the photoperiodic circuit continues to circulate at a constant intensity. However, the amplification rate of the rare-earth-doped fiber 1 varies according to the intensity of the circulating light.
The reason that the amplification rate of the rare-earth-doped fiber 1 varies according to the intensity of the circulating light is explained below. FIG. 8 is a diagram showing an optical amplifier using a rare-earth-doped fiber 1 for the case in which the intensity of the excitation light is constant. The rare-earth-doped fiber 1 converts and accumulates the excitation light outputted from the excitation light source 10, and goes into an excited state. Then, if an optical signal is inputted from the beam input end 11, the rare-earth-doped fiber 1 which is in an excited state converts the accumulated excitation energy into an optical signal having the same wavelength as the inputted optical signal. Furthermore, by releasing the converted optical signal, the rare-earth-doped fiber 1 amplifies the inputted optical signal. The amplification rate of the rare-earth-doped fiber 1 depends upon the amount of accumulated energy.
Therefore, if an optical signal with a waveform as shown in FIG. 9A is inputted to the ram-earth-doped fiber 1 excited by excitation light of a constant intensity through the light input end 11, the output obtained at the light output end 12 has a waveform as shown in FIG. 9B. From the outputted waveform shown in FIG. 9B, it is apparent that directly after the optical signal is inputted, there is a momentary increase in the optical amplification rate of the rare-earth-doped fiber. This phenomenon arises because, before the optical signal is inputted, the excitation light inputted from the excitation light source 10, converted to excitation energy remains unused, and therefore accumulates within the rare-earth-doped fiber 1.
Then, as shown in FIG. 9B, after the momentary increase in the optical signal outputted from the rare-earth-doped fiber 1, the intensity of said optical signal begins to decline, and eventually a constant output signal remains. This is because the excitation energy used by the rare-earth-doped fiber 1 is greater than the energy of the excitation light received from the excitation light source 10, and consequently the amplification rate of the rare-earth-doped fiber 1 gradually decreases. Then, as the amplification rate of the rare-earth-doped fiber 1 decreases, the intensity of the optical signal outputted by the rare-earth-doped fiber 1 decreases, and the amount of excitation energy used by the rare-earth-doped fiber 1 becomes low.
Then, when the amount of energy from the excitation light supplied by the excitation light source 10 matches the amount of excitation energy used up by the amplification, the amount of excitation energy accumulated within the rare-earth-doped fiber 1 becomes constant. The amplification rate of the rare-earth-doped fiber 1 depends upon the amount of excitation energy within the rare-earth-doped fiber 1. As a result, when the amount of excitation energy accumulated within the rare-earth-doped fiber becomes constant, the amplification rate of the rare-earth-doped fiber becomes constant as well. The amount of time required for the amplification rate of the rare-earth-doped fiber 1 to become constant may vary from a few microseconds to a few milliseconds.
Next, the variation in the intensity of the light circulating in the photoperiodic circuit 103 shown in FIG. 7 will be explained with reference to FIGS. 10A-10D.
FIG. 10A: If the intensity of the excitation light inputted to the rare-earth-doped fiber 1 is weak, then the amplification rate of the photoperiodic circuit 103 does not exceed "1". Therefore, the intensity of the circulating light is attenuated according to the number of circuits made by the light circulating within the photoperiodic circuit 103.
FIG. 10B: As the output of the excitation light source drive circuit 9 is increased, the intensity of the excitation light supplied to the rare-earth-doped fiber 1 increases, and the intensity of the circulating light begins to fluctuate. Then, the intensity of the circulating light is attenuated as it repeatedly goes up and down.
FIG. 10C: As the intensity of the excitation light in the rare-earth-doped fiber further increases, the circulating light continues to circulate while repeating mild fluctuations.
FIG. 10D: If a pulse train of optical signals is inputted to an optical amplification circuit (shown in FIG. 5) which does not have a photoperiodic circuit, then circulating light as shown in FIG. 7D is outputted.
Next, the reason that the intensity of the light circulating in the photoperiodic circuit shown in FIG. 7 fluctuates as shown in FIG. 10C will be explained with reference to FIG. 11. The dotted line L1 shown in FIG. 11A describes the intensity of the circulating light when the excitation energy accumulated in the rare-earth-doped fiber 1 and the excitation energy used by the rare-earth-doped fiber 1 are equal. When the excitation energy accumulated in the rare-earth-doped fiber 1 and the excitation energy used by the rare-earth-doped fiber become equal, the amplification rate of the photoperiodic circuit 103 becomes "1". Additionally, the dotted line L.sub.2 in FIG. 11B describes the amplification rate of the photoperiodic circuit when it is constant at "1".
First, when the photoperiodic circuit 103 is cut off by the optical switch 3, the circulating light is turned off, and the circulation activity of the present circulating light ends. Afterwards, until the photoperiodic circuit 103 is reconnected by the optical switch 3 and the next optical signal is inputted, no optical signal is inputted to the rare-earth-doped fiber 1. However, even in the state in which no optical signal is inputted, excitation light is still being transmitted from the excitation light source 10, so the rare-earth-doped fiber continues to convert and store excitation energy. As the rare-earth-doped fiber 1 continues to convert and store the excitation energy, the amplification rate of the photoperiodic circuit 103 increases (Time A in FIG. 11). As a result, by the time the next optical signal is inputted from the light input end 7, the amplification rate of the photoperiodic circuit 103 is greater than "1" (Time B in FIG. 11). Because the amplification rate of the photoperiodic circuit is greater than "1", the intensity of the circulating light increases each time the circulating light completes a circuit (Time C in FIG. 11). However, since the excitation light transmitted from the excitation light source 10 to the rare-earth-doped fiber 1 is constant, the excitation energy expended by the rare-earth-doped fiber for the amplification of the circulating light is greater than the excitation energy newly generated from the excitation light. Therefore, at time C in FIG. 11A, the intensity of the circulating light increases, but the amplification rate of the photoperiodic circuit 103 decreases.
When the amplification rate of the photoperiodic circuit 103 becomes less than "1", the intensity of the circulating light begins to be attenuated (Time D in FIG. 11) . However, since the intensity of the circulating light is above the dotted line L.sub.1, the excitation energy used for the amplification of the circulating light is greater than the excitation energy generated from the excitation light. Consequently, the decrease in the amplification rate of the photoperiodic circuit 103 continues (Time E in FIG. 11). Eventually, when the intensity of the excitation light goes below the dotted line L.sub.1, the excitation energy used for the amplification of the circulating light becomes smaller than the excitation energy generated from the excitation light. As a result, the amplification rate of the photoperiodic circuit 103 begins to increase (Time F in FIG. 11). However, since the amplification rate of the photoperiodic circuit 103 remains below "1", the intensity of the circulating light continues to decrease (Time G in FIG. 11). Then, when the amplification rate of the photoperiodic circuit exceeds "1", the intensity of the circulating light begins to increase. At this time, since the intensity of the circulating light is not above the dotted line L.sub.1, the amplification rate of the photoperiodic circuit 103 continues to increase (Time I in FIG. 11).
As explained above, the intensity of the circulating light and the amplification of the photoperiodic circuit 103 am mutually influenced by each other. Specifically, while the intensity of the circulating light is above L.sub.1, the amplification of the photoperiodic circuit 103 decreases, and while the amplification rate of the photoperiodic circuit 103 is above L.sub.2, the intensity of the circulating light increases. As a result, as shown in FIG. 11A, the intensity of the circulating light repeatedly goes up and down, and does not stay constant at L.sub.1.
In order to solve the problem described above, in the past, through negative feedback control of the amplification rate of the rare-earth-doped fiber 1, the intensity of the circulating light was held constant at L.sub.1 as shown in FIG. 11A. Below, this amplification control apparatus will be explained with reference to FIG. 12. As shown in the diagram, with this amplification control apparatus, excitation light is supplied to the rare-earth-doped fiber from a monitored excitation light source 13 through a wavelength multiplexing light mixer/divider 2. Then, the amplification rate of the ram-earth-doped fiber 1 is controlled by this excitation light and the excitation light which has passed through the rare-earth-doped fiber 1. The excitation light which passed through the rare-earth-doped fiber 1 is called leakage light.
Since a monitored excitation light source 13 is used as a light source for excitation light, an electrical signal proportional to file supplied excitation light is detected by the monitor output a. Simultaneously, since a wavelength multiplexing light mixer/divider 14 is provided, reversals in the photoperiodic circuit 104 due to leakage light are prevented. Additionally, the wavelength multiplexing light mixer/divider 14 outputs said leakage light to a light-receiving element 15. Thereby, an electrical signal proportional to the intensity of the leakage light of the rare-earth-doped fiber 1 is detected by the light-receiving element 15. Then, the division circuit 16 divides the electrical signal detected by the light-receiving element 15 by the electrical signal detected by the monitored excitation light source 13.
Additionally, the logarithmic amplification circuit 17 makes a logarithmic conversion of the results of the division due to the division circuit 16. This logarithmically convened value (that is, log [leakage light intensity/excitation light intensity]) is proportional to the amplification rate of the rare-earth-doped fiber 1. Furthermore, the logarithmic amplification circuit 17 reverses the positive/negative polarity of the logarithmically converted value. The addition circuit 18 adds the output of the logarithmic amplification circuit 17 to the output of the standard power source 19, and supplies the result to the monitored excitation light source 13. The monitored excitation light source 13 controls the intensity of the excitation light based on the output of the addition circuit 18. In this way, through negative feedback control of the output of the rare-earth-doped fiber 1, the conventional amplification control apparatus maintained the amplification rate of the photoperiodic circuit at "1".
FIG. 13 is a diagram showing the flow of the signal of the amplification control apparatus shown in FIG. 12. In the diagram, the arrow AR indicates the signal flow. Additionally, in this diagram, the symbols. (+) and (-) refer to the polarity of the signal. In order for this amplification control apparatus to operate normally, the response speed of the signal of the feedback route (arrow AR.sub.1) entering the division circuit 16 from the monitor output a of the monitored excitation light source 13 and the response speed of the signal of the feedback route (arrow AR.sub.2) entering the division circuit 16 from the light-receiving element 15 must be approximately equal. The following explanation assumes that the response speed of the signal of the feedback route arrow AR.sub.1 and the response speed of the signal of the feedback route arrow AR.sub.2.
It is assumed that a voltage having positive polarity is applied to an inputted electrode b of the addition circuit 18. The output of the addition circuit 18 increases, and as a result the output of the monitored excitation light source 13 also increases. Then, when the output of the monitored excitation light source 13 increases, the monitor output a increases. The monitor output a of the monitored excitation light source 13 is substituted into the divisor of the division made by the division circuit 16.
Simultaneously, the excitation light from the monitored excitation light source 13 inputted to the rare-earth-doped fiber 1 turns into excitation energy within the rare-earth-doped fiber 1. Then, as the excitation energy in the rare-earth-doped fiber 1 increases, the leakage light also increases. The increased leakage light is supplied to the light-receiving element 15 through the wavelength multiplexing light mixer/divider 14. Consequently, the output of the light-receiving element increases. This output of the light-receiving element 15 is inputted to the numerator side of the division circuit 16.
The division circuit 16 outputs an electrical signal indicating the value resulting from the division of the output of the light-receiving element 15 by the monitor output a of the monitored excitation light source 13. At this time, since the proportional increase in the output of the light-receiving element 15 becomes larger than the proportional increase in the monitor output a of the monitored excitation light source 13, the output of the division circuit 16 increases. The logarithmic amplification circuit 17 logarithmically converts the output of the division circuit 16. Furthermore, the logarithmic amplification circuit 17 reverses the polarity of the logarithmically converted value and outputs the result to the addition circuit 18. Therefore, if the output of the division circuit 16 increases, then the output of the logarithmic amplification circuit 17 decreases. As a result, a positive voltage is applied to the input electrode b of the addition circuit 18, and when the excitation light outputted by the monitored excitation light source 13 increases, a negative voltage is applied to the input electrode b of the addition circuit 18 by the above-mentioned negative feedback control.
If the circulation period of the circulating light is more than a few hundred microseconds, fluctuations in the intensity of the circulating light are somewhat suppressed by negative feedback control of the conventional amplification control apparatus. However, if the circulation period of the circulating light becomes any shorter, since the response speed of the above-described amplification control apparatus is slow, it becomes difficult to suppress fluctuations of the circulating light. The reason that the response speed of the above-described amplification control apparatus is slow will be explained below.
First, in order for the rare-earth-doped fiber 1 to function as an amplifier, it is necessary for the rare-earth elements within the rare-earth-doped fiber 1 to be in an excited state. Thus, when excitation light is injected into the rare-earth-doped fiber 1, the optical energy of this excitation light causes the rare-earth elements within the rare-earth-doped fiber 1 to become excited. However, from a few hundred microseconds to a few milliseconds are necessary for the rare-earth elements to reach an excited state after the excitation light is injected into the rare-earth-doped fiber 1. As a result, the change in the amplification rate of the rare-earth-doped fiber 1 trails the change in intensity of the excitation light.
FIG. 14 is a diagram showing the relationship between the modulation frequency of the excitation light and the variation in the amplification rate of the rare-earth-doped fiber 1 when the intensity of the excitation light is modulated by a sine wave. With the above-mentioned amplification control apparatus, if the electric circuit portion becomes high-speed, then the intensity of the circulating light fluctuates. Similarly, with the above-mentioned amplification control apparatus, if the amount of negative feedback is increased, then the intensity of the circulating light fluctuates. Consequently, it is not possible to sufficiently suppress fluctuations of the circulating light. The reason that the intensity of the circulating light fluctuates is given below.
FIG. 15A is a diagram showing the signal flow in the amplification control apparatus when the electric circuit portion is high-speed. In the diagram, the response speed of the feedback route AR.sub.1 is significantly faster than the response speed of the feedback route AR.sub.2. In this case, as seen in the FIG. 15A, because the route (arrow AR.sub.1) described by addition circuit 18.fwdarw.monitored excitation light source 13.fwdarw.division circuit 16.fwdarw.logarithmic amplification circuit 17.fwdarw.addition circuit 18 operates as a positive feedback route, the intensity of the circulating light fluctuates. The reason is as follows.
In this case, it is assumed that a positive voltage is inputted to the input electrode b of the addition circuit 18. Then, the output value of the monitor output a of the monitored excitation light source 13 and the output value of the light-receiving element 15 are inputted to the division circuit 16. As mentioned above, the output value of the monitor output a is substituted into the divisor, and the output of the light-receiving element 15 is substituted into the numerator. At this time, since the amplification rate variation of the rare-earth-doped fiber 1 is gentle, the speed of increase of the leakage light is slow in comparison with the speed of increase of the output value of the monitor output a of the monitored excitation light source 13. As a result, the speed of increase of the output value of the light-receiving element 15 is slow in comparison with the speed of increase of the output value of the monitor output a of the monitored excitation light source 13.
Consequently, of the signals inputted to the division circuit 16, only the monitor output a of the monitored excitation light source 13 significantly increases. As a result, for the division carded out by the division circuit 16, since only the monitor output a of the monitored excitation light source 13 (the divisor in the division) significantly increases, the output of the division circuit 16 decreases. When the output of the division circuit 16 decreases, the output of the logarithmic amplification circuit 17 increases, and the positive voltage applied to the addition circuit also increases.. Therefore, when the electric circuit portion is made high-speed, the amplification control apparatus becomes a positive feedback system, and the intensity of the circulating light fluctuates.
FIG. 15B is a diagram showing the signal flow when the amount of feedback of the amplification control is increased. If the amount of feedback for negative feedback control is decreased, errors in the negative feedback route become rarer, and it is possible to suppress fluctuations of the circulating light. However, even in this case, the control operation may become unstable or the circulating light may fluctuate. This is because the signal of the feedback route (arrow AR.sub.2) passing through the light-receiving element is delayed and the phase of the signal changes, and as a result, the feedback route passing through the light-receiving element becomes a positive feedback route.
In this case, when the signal variation is slow, the signal outputted by the light-receiving element 15 has a phase which trails that of the excitation light output of the monitored excitation light source 13 by 90 degrees. This is because the leakage light inputted to the light-receiving element 15 becomes the excitation energy stored in the rare-earth-doped earth-doped fiber 1. Thus, the output of the light-receiving element 15 becomes the value of the integral of the monitor output of the monitored excitation light source 13. When the signal variation becomes fast, the output of the light-receiving element 15 is delayed by the time it takes to complete the route (arrow AR.sub.2) described by monitored excitation light source 13.fwdarw.wavelength multiplexing light mixer/divider 2.fwdarw.rare-earth-doped fiber 1.fwdarw.wavelength multiplexing light mixer/divider 14.fwdarw.light-receiving element 15. Therefore, when the operation speed is fast, the delay due to propagation time is added to the 90 degree phase delay due to the integration operation, and what was supposed to be a negative feedback control system becomes a positive feedback control system. If the amount of feedback is small, the feedback amplification factor for fast operation speeds becomes less than 1, and the intensity of the circulating light becomes stable. However, if the amount of feedback is large, then the feedback amplification factor for fast operation speeds increases, and if the feedback amplification factor for positive feedback becomes greater than 1, the intensity of the circulating light fluctuates.
As explained above, the conventional amplification control apparatus can stably perform negative feedback control as long as it is operating at a relatively slow speed. However, if the operation is made high-speed or the amount of feedback is increased, then the negative feedback control system of the amplification control apparatus changes to a positive feedback control system. As a result, with a conventional amplification control apparatus, if the response speed of the electrical circuit portion of the amplification control apparatus is made faster or the amount of negative feedback is increased, then the control of the amplification control apparatus becomes unstable, and the intensity of the circulating light fluctuates.
Additionally, as a different way to hold the intensity of the circulating light constant, a method whereby the intensity of the circulating light is measured and negative feedback is sent to the excitation light source itself may be considered. However, as seen in FIG. 11, because the change in the intensity of the circulating light is delayed in comparison with the amplification rate of the photoperiodic circuit, fluctuations of the circulating light are made worse by this type of simple negative feedback control (see FIG. 16).
The present invention was made in light of a background which suffered from these types of problems, and has as an objective the presentation of an amplification control apparatus for a photoperiodic circuit which is able to suppress fluctuations in the circulating light.